1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus for printing images on papers, OHP (OverHead Projector) films or similar sheets. More particularly, the present invention relates to a fixing device arranged in such an image forming apparatus and including a heat roller or similar rotatable heating member and a press roller or similar rotatable pressing member configured to fix an image formed on a sheet with heat and pressure while conveying the sheet in cooperation.
2. Description of the Background Art
An electrophotographic image forming apparatus including a fixing device of the type described is conventional. In this type of fixing device, if heat generated by a heat roller or similar rotary heat-generating member leaks, then not only energy necessary for fixation and therefore cost increases, but also the warm-up time of the fixing device increases. Further, heat leaked from the heat roller heats members other than the fixing device to thereby bring about toner filming, adhesion, deterioration of a developer and other problems.
In light of the above, Japanese Patent Laid-Open Publication No. 2000-221824, for example, discloses an image forming apparatus including a fixing device in which a heat-resistant elastic layer is formed on the metallic core of a press roller. This elastic layer has thermal conductivity of 0.1 W/m·k or below. The problem with the above fixing device is that because heat insulation is degraded by the compressive deformation of the elastic layer, the elastic layer must be provided with certain thickness, resulting in an increase in roller diameter and therefore in the overall size of the fixing device.
On the other hand, Japanese Patent Laid-Open Publication No. 2000-29342, for example, teaches a fixing device in which ribs are formed on the inner periphery of a press roller in order to reduce the wall thickness and therefore thermal capacity of the heat roller, thereby accelerating the warm-up of the fixing device. The thin heat roller, however, gives rise to another problem that pressure for fixation cannot be made high enough to meet the increasing demand for high-speed fixation.
In a conventional fixing device configured to implement high-speed fixation, a heat-generating layer is formed on a heat roller for reducing a roller diameter. In addition, a metallic core is positioned inward of the heat-generating layer in order to protect the heat-generating layer from deformation even when pressure for fixation is increased. The metallic core, however, absorbs heat output from the heat-generating layer and thereby slows down the temperature elevation of the heat-generating layer. This again brings about the various problems ascribable to the leak of heat stated earlier.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 9-22208 and 11-162299 and Japanese Patent Laid-Open Publication Nos. 2000-29328, 2000-347524, 2001-5315, 2001-32825 and 2002-40855.